1. Field of the Invention
This invention relates to apparatus and a method for storing analog signals for prolonged periods of time. More particularly, it relates to a method and apparatus for long-term storage of an approximation of an analog signal which is periodically refreshed to maintain its value within a prescribed window. Thousands of the devices with which the invention is implemented can be provided on a single semiconductor chip.
2. Background Information
Storage of a variable analog signal in computing circuits is often implemented by sample and hold circuits which store an instantaneous value of the analog signal as a voltage on a capacitor. However, such devices by themselves are not useful for long-term storage because the voltage droops as a result of leakage of the charge stored in the capacitor.
Longer term storage of analog signals can be achieved by converting the analog signal to a digital signal which can be stored indefinitely, and then converting the stored digital signal back to an analog signal when it is needed. However, such a scheme requires analog to digital and digital to analog convertors, and considerable real estate to implement. This becomes a significant problem when a large number of analog signals must be stored for a prolonged period. In addition, the analog to digital and reverse conversions require a considerable amount of time which could be significant in some applications.
Of course, analog signals can be stored indefinitely on magnetic media; however, access time is unacceptably high for massive computations where the stored values are continually changing and/or re-used frequently.
One application where prolonged storage of analog signals is required is in what we call a "state-analog" system in analogy to a state logic system. The digital computer utilizes state logic. State logic is discrete time, discrete-voltage logic which in its purest form has the following constraint: all signals which are inputs for computation are acquired by computing elements at specific times, determined by a clock. The time of acquisition is a time when all the signals are known to be the final values obtained in the previous compute cycle, and before the initiation of the next compute cycle. This generally means that the outputs of computing elements are captured at clock transitions, and held stable so that the computing elements can perform their next computations on stable values. This is necessary because there often exist feedback or feedforward paths which otherwise would cause inputs to a computing element to change while it is performing its computation. In parallel computing applications, such as for example neural nets, it is quite common for the outputs of computing elements to influence each other, and some method of "freezing" results of one computation step is needed so that the next computation step can be performed on a stable input value. In many instances, it is necessary to use a two-phase clocking system with the equivalent of master-slave flipflops used to capture signals. Captured signal values are said to go through successive "states" on successive clock cycles, as do all signal values: hence the name "state logic".
In what we call "state analog" devices, the computing elements are continuous-voltage (analog) devices, but a clocking scheme is used to capture results of computations, and such a device is therefore a state machine. The sample and hold circuit could be considered a state analog device in that the signal to be stored is sampled by a clock pulse, except that, as mentioned above such devices are not suitable for long-term storage of analog signals.
There remains therefore a need for a method and apparatus for long-term storage of variable analog signals.
Accordingly, it is a primary object of this invention to provide an improved method and apparatus for prolonged storage of analog signals.
It is another important object of the invention to provide a state-analog method and apparatus for prolonged storage of analog signals.
It is an additional object of the invention to provide the method and apparatus of the previous objects which can realize high density storage of analog signals, and which can operate reliably at high clock frequencies.